Recently, read-only optical disks and write-once type optical disks have been put into practice, and rewritable optical disks are also going to be put into practice. In any of these optical disks, a track pitch is typically very narrow at about 1-2 .mu.m, and therefore concave or convex pits or grooves for performing track tracking are formed in each disk in advance. The relative positional relationship between a track and an information reading beam spot in the radial direction of the disk can be detected on the basis of diffraction of light radiated onto and reflected from the disk by means of the pits or grooves, whereby it is possible to form a tracking servo for making the beam spot follow the track. Further, the pits can be used as information to support ancillary tasks, i.e., for generating a clock required for recording/reproducing data, information for dividing a track into sectors, information for providing access to a sector, information for dividing the inside of a sector into blocks, and the like, and the various information is read by diffraction of light due to the pits. Such pits formed in the disk in advance for the purpose of obtaining information by using diffraction of light as described above are called embossed pits.
FIGS. 3 to 5 show an example of the arrangement of embossed pits (a so-called format) in a disk.
In the format shown in FIGS. 3 to 5, a track formed virtually spirally in the disk is divided into 1376 equiangular segments for every revolution. Further, 43 successive segments constitute a sector so that a full revolution of a track is constituted by 32 sectors.
FIG. 3 shows the segment configuration of each sector. Each segment is constituted by 18 bytes in which 2 bytes constitute a servo area and 16 bytes constitute a header area or a data area. The first segment has a header area of 16 bytes, and each of the second to forty-third segments has a data area of 16 bytes. Each cf the bytes constituting the servo area, the header area, and the data area is divided into 15 channel bits.
FIG. 4 is a diagram showing the configuration of the servo area. The servo area in each segment is constituted by 2 bytes. The bytes constituting the servo area are called first and second servo bytes respectively. Two embossed bits are formed in the first servo byte. The embossed bits are formed in positions each radially displaced (i.e., wobbled) from the virtual track center by about a 1/4 track pitch in opposite directions with respect to each other.
The first wobbled pit PW1 is formed at a selected one of the positions of the third and fourth channel bits while the positions are alternatively changed over for every 16 tracks, and the second wobbled pit PW2 is formed at the position of the eighth channel bit. A tracking error signal can be generated once for every segment by means of the two wobbled pits in a sampling manner. That is, since a beam spot passes a middle position between the two wobbled pits when it passes the virtual track center, the degrees of diffraction in the respective wobbled pits are equal to each other and the respective intensities of reflected light thereat are equal to each other. As a result, a tracking error signal formed on the basis of a difference between signals obtained by photoelectrically converting the respective intensities of reflection light is zero (indicating no error). If the beam spot passes a position displaced from the virtual track center, on the contrary, a difference is generated between the respective intensities of reflected light from the two wobbled pits so that a tracking error signal corresponding to the direction and quantity of the displacement is obtained. Since 1376 segments exist in one revolution, the tracking error signals obtained in a sampling manner in the servo bytes are substantially equivalent to those obtained continuously, thereby making it possible to perform tracking servo.
Further, in the second servo byte, one embossed pit is formed just at the virtual track center in the position of the twelfth channel bit. This embossed pit is called a clock pit PC. Since one clock pit PC is formed every segment at a fixed position in the servo bytes therein, it is possible to generate a clock having a frequency equal to the channel bit rate by synchronizing a PLL with signals reproduced at predetermined intervals. Modulation is performed on the basis of this clock at the time of data recording, while demodulation is performed also on the basis of this clock at the time of data reproducing.
Since a mirror surface is provided between PW2 and the PC, it is possible to generate a stable focus error signal in a sampling manner without being affected by the existence of the pits.
Further, since the interval between the PW2 and the PC is selected to be a value (19 channel bits) which cannot appear in a 4/15 modulation system described later, it is possible to perform segment synchronization by detecting the interval.
FIG. 5 shows the configuration of the header area. In the first byte, synchronizing marks are formed by means of embossed pits. That is, to form the synchronizing marks, the embossed pits are formed in the second, seventh, eighth, and ninth channel bits, resulting in a peculiar pattern which does not correspond to any NRZ data in a conversion table of the 4/15 modulation system which will be described later. Thus, it is possible to perform sector synchronization by detecting this pattern. In the second byte, a sector address in each track is formed by embossed pits. In the third to seventh bytes, a track address in the disk is formed by embossed pits. The addresses are modulated for every byte in accordance with the 4/15 modulation system which will be described later. The eighth to thirteenth bytes form a reserve area for a non-determined use, the reserve area being formed by a mirror surface having no embossed pits. The fourteenth to sixteenth bytes form a laser power control area which is formed by a mirror surface initially having no embossed pits. Although it is desirable to use suitable light power in the case of performing recording/erasing on a disk, it is permissible in this area that recording/erasing power is experimentally emitted from an optical pickup so that emission power is corrected on the basis of experimental power emission.
Further, the data area has a length of 16 bytes and is formed by a mirror surface having no embossed pits in a non-recorded state. NRZ data are modulated for every byte on the basis of the 4/15 modulation system which will be described later, and the modulated NRZ data: are recorded in the area. In the case of a write-once type optical disk, recording is accompanied by a physical change such as formation of holes in a recording film. In the case of a rewritable disk utilizing a photomagnetic effect (hereinafter, referred to as a photomagnetic disk), recording is accompanied with no such physical change but is accompanied by a change such as inversion of the polarity of the magnetic field on the disk.
Each sector has 43 segments, and 42 segments of them have the data area, respectively. Each segment including the data area has 16 bytes so that each sector has (16 bytes.times.42 sectors=672 bytes) 672 bytes which are constituted by user's data, an error correcting code, and the like. Description as to the data area is not made in detail here.
Next, referring to FIG. 6, description will be made as to the 4/15 modulation system. In the 4/15 modulation system, one byte is converted into 15 channel bits, and marks are recorded on four portions (two odd numbered channel bit portions and two even numbered channel bit portions, except the fifteenth channel bit portion) of the 15 channel bit portions with one-to-one correspondency with the original 256 kinds of NRZ data in the conversion table. That is, for example, operation for formation of holes in a recording film is performed in the case of a write-one type optical disk, and operation for inversion of the direction of magnetization of a recording film is performed in the case of a photomagnetic disk. Although marks may be adjacent to each other (at the twelfth, thirteenth, and fourteenth channel bit positions) as shown in the example of FIG. 6, it must be selected that the interval between two marks (the ninth and twelfth channel bit positions) which are not adjacent to each other corresponds to 2 channel bits or more (the tenth and eleventh channel bits). Exceptionally, there occurs a case where marks are formed at the fourteenth channel bit position in a certain byte and at the first channel bit position in the next byte, and the interval between the two marks is formed only by one channel bit portion (the fifteenth channel bit). In this case, however, no mark is ever formed at the fifteenth channel bit position originally, and therefore the foregoing case does not give a harmful influence at the time of demodulation.
Next, description will be made as to demodulation of data in accordance with the 4/15 modulation system. FIG. 6 shows a reproduced waveform corresponding to the marks. In the case of performing recording by formation of holes, the intensity of the reflection light at a mark position is weaker than that at a no mark position (at a mirror surface). In media of the type of performing recording not by formation of holes, there are some cases in which changes occur in a manner opposite to the foregoing case. According to the 4/15 modulation system, however, demodulation can be performed so long as a level difference exists between the mark position and the mirror surface. It is therefore assumed that the reproduced waveform of FIG. 6 does not show the fact that the upper portion in the drawing is bright but merely shows a voltage level at a certain position in a demodulation circuit. In the case of a photomagnetic disk, the above-mentioned mirror surface level is replaced by an erase level. Demodulation can be performed so long as marks positioned at two odd numbered channel bits and two even numbered channel bits in the first to fourteenth channel bits in a certain byte can be identified. The mark positions can be identified, for example, if A/D conversion is performed at the bit center of the first to fourteenth channel bits and values of the obtained digital data are compared with each other. For example, in the example of FIG. 6, among the first, third, fifth, seventh, ninth, eleventh, and thirteenth channel bits, the thirteenth channel bit has the highest level, and the ninth channel bit has the second higher level. (In this example, the level of the fifteenth channel bit is sometimes higher than that of the ninth channel bit because the marks are formed at the fourteenth channel bit and the first channel bit of the next byte. No mark, however, is ever formed at the fifteenth channel bit position and therefore the level of the fifteenth channel bit is not made to be a subject for comparison. Accordingly, there occurs no harmful influence on demodulation). That is, it is understood that marks are formed at the ninth and thirteenth ones of the odd numbered channel bits, and, similarly, marks are formed at the twelfth and fourteenth ones of the even numbered channel bits. It is possible to demodulate the original NRZ data from the marks at the above-mentioned four positions by using the conversion table.
In short, in demodulation of the 4/15 modulation system, it is fundamental to compare the reproduction levels at the centers of the respective channel bits to each other.
FIG. 7 shows a recorded data demodulation circuit for demodulating data recorded on an optical disk as described above.
In FIG. 7, a so-called RF (high frequency) signal a which is a read signal produced from a pickup (not shown) is supplied to a demodulating clock generation circuit 1 and a transversal filter 2. The demodulating clock generation circuit 1 is configured to generate, as a demodulating clock ck, a clock of a predetermined frequency having a leading edge synchronized, for example, with a peak level point of the RF signal a.
In the transversal filter 2, the RF signal a is delayed by a predetermined time successively by means of analog delay elements 3 and 4 such as delay lines or the like, and the delayed RF signal is supplied to an addition/subtraction circuit 6 through an amplifier 5. Further, the RF signal and the output of the analog delay element 3 are supplied to the addition/subtraction circuit 6 through amplifiers 7 and 8, respectively. In the addition/subtraction circuit 6, a signal is obtained by subtracting the respective outputs of the amplifiers 5 and 7 from the output of the amplifier 8. The output of the addition/subtraction circuit 6 is supplied, as the output of the transversal filter 2, to an analog-to-digital (hereinafter, referred to as "A/D") conversion circuit 9.
In the A/D conversion circuit 9, the instantaneous level of the RF signal a is sampled, for example, in response to the leading edge of the demodulating clock ck, and digital data of n bits (n is a natural number) corresponding to the obtained sampling value are generated. The output data of the A/D conversion circuit 9 are supplied to a demodulation circuit 10 so as to be subject to demodulation processing, for example, in accordance with the 4/15 modulation system.
In the thus configured transversal filter 2, let the gain of each of the amplifiers 5 and 7 be represented by K.sub.1, the gain of the amplifier 8 be represented by K.sub.0, and the delay time of each of the analog delay elements 3 and 4 be represented by T. Then, the frequency characteristic G(j.omega.) of the transversal filter 2 is expressed by the following equation (1). ##EQU1##
In the expression (1), e.sup.-j.omega.T represents existence of a predetermined delay, and (K.sub.0 -2K.sub.1 cos.omega.T) represents the gain.
Therefore, the gain of the transversal filter 2 represented by a graph as shown in FIG. 8. By the transversal filter 2, the reduction in level of the components existing in a frequency band around a frequency of 1/2T is compensated to thereby perform waveform equalization, that is, waveform processing for shaping the waveform of the RF signal to be a waveform approximate to that of the original recorded signal. As a result, erroneous demodulation in the demodulation circuit 10 is prevented so that the error rate can be improved.
In the recorded data demodulation circuit as described above, however, there has been a disadvantage in that the equalization characteristics fluctuate because of scattering of the analog delay elements or scattering of the amplifier gains and there occurs a case where adjustment operation is required. Further, since it is necessary to change the gains of the amplifiers in the case where the equalization characteristics are to be changed, it has been difficult to improve the error rate by changing the equalization characteristics. Moreover, in the recorded data demodulation circuit, there has been a further disadvantage in that, since the analog delay elements are parts which are expensive and which are difficult to be constituted in the form of an IC, the demodulation circuit is high in manufacturing cost and is difficult to be constituted in the form of an IC.